Many applications such as medical imaging and cellular radios need to manage signal amplitudes. Many applications require both analog and digital signal processing to provide the best combination of cost and performance. In such cases, a VGA (variable-gain amplifier) allows a product to adapt to changeable operating environments and excessive signal dynamics.
A wideband VGA can be implemented by following a voltage-controlled-attenuator block by a gain cell. The attenuator is constructed in MOS with an offset cascade of amplifiers driving the gates of attenuator FET transistors. The magnitude of the VCA input signal (from the signal source or preamplifier) is reduced by a programmable attenuation factor, set by the analog VCA control voltage.
FIG. 1 illustrates this adjustable characteristic. The analog variable gain characteristic is linear in dB as a function of the control voltage, and is created as a piecewise approximation of an ideal dB-linear transfer function. Internally, the signal is attenuated by having the analog control voltage vary the channel resistance of a set of shunt-connected FET transistors.
Referring to FIG. 2, a prior art variable gain amplifier (VGA) is shown. The analog VCA control voltage gain setting is accomplished by multiple FET shunt elements. The attenuator control stage including differential amplifiers A1-AK and the offset generator produces K output voltages V1-VK in which the i-th output is equal to VCM+VT when control input is equal to Vi, and varies as a function of the threshold voltage VT of the N-channel MOSFET of the integrated circuit on which the logarithmic attenuator is formed. This type of VCA has been used in some variable-gain amplifier chips. However, the attenuation can be quite process-dependent and unstable.
Thus, an improved apparatus and method that overcomes the above problems is desire. The improved apparatus and method must be able to match the on-resistance of Q1-K to resistors R1-K and RS to provide a high-performance VGA circuit.